Metal gate structure and method of formation

ABSTRACT

Embodiments of the present invention provide a metal gate structure and method of formation. In the replacement metal gate (RMG) process flow, the gate cut process is performed after the metal gate is formed. This allows for a reduced margin between the end of the gate and an adjacent fin. It enables a thinner sacrificial layer on top of the dummy gate, since the gate cut step is deferred. The thinner sacrificial layer improves device quality by reducing the adverse effect of shadowing during implantation. Furthermore, in this process flow, the work function metal layer is terminated along the semiconductor substrate by a capping layer, which reduces undesirable shifts in threshold voltage that occurred in prior methods and structures.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor fabrication,and more particularly, to a metal gate structure and method offormation.

BACKGROUND

As technology dimensions scale for fin type field effect transistors(finFETs), the gate end region beyond the fin (gate endcap) and layerthicknesses for gate cap and spacers need to scale accordingly. However,as critical dimensions get smaller, these features create challengesthat can adversely impact circuit density, product yield, and devicevariability. It is therefore desirable to have improvements in metalgate structures and methods of formation.

SUMMARY

Embodiments of the present invention provide a metal gate structure andmethod of formation. In the replacement metal gate (RMG) process flow,the gate cut process is performed after the metal gate is formed. Thisallows for a reduced margin between the end of the gate and an adjacentfin. It enables a thinner sacrificial layer on top of the dummy gate,since the gate cut step is deferred. The thinner sacrificial layerimproves device quality by reducing the adverse effect of shadowingduring implantation. Furthermore, in this process flow, the workfunction metal layer is terminated along the semiconductor substrate bya capping layer, which reduces undesirable shifts in threshold voltagethat occurred in prior methods and structures.

In a first aspect, embodiments of the present invention provide a methodof forming a semiconductor structure, comprising: forming a dummy gateon a semiconductor substrate; depositing a sacrificial layer on thedummy gate; forming spacers adjacent to the dummy gate; removing thedummy gate and sacrificial layer; depositing a work function metallayer; depositing a fill metal layer; performing a gate cut; anddepositing a gate capping layer.

In a second aspect, embodiments of the present invention provide asemiconductor structure, comprising: a semiconductor substrate; a finformed on the semiconductor substrate; a gate stack formed over the fin,wherein the gate stack comprises: a work function metal layer disposedon the fin; a metal fill layer disposed on the work function metallayer; and a capping layer disposed on a top and side portions of thegate stack, wherein the work function metal layer terminates at thecapping layer on the semiconductor substrate.

In a third aspect, embodiments of the present invention provide asemiconductor structure, comprising: a semiconductor substrate; a finformed on the semiconductor substrate; a gate stack formed over the fin,wherein the gate stack comprises: a work function metal layer disposedon the fin; a metal fill layer disposed on the work function metallayer; and a capping layer disposed on a top and side portions of thegate stack, wherein the work function metal layer terminates at thecapping layer on the semiconductor substrate, and wherein the cappinglayer disposed on the side portions of the gate stack is disposed at adistance from the fin ranging from about 5 nanometers to about 15nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thepresent teachings and, together with the description, serve to explainthe principles of the present teachings.

Certain elements in some of the figures may be omitted, or illustratednot-to-scale, for illustrative clarity. The cross-sectional views may bein the form of “slices”, or “near-sighted” cross-sectional views,omitting certain background lines which would otherwise be visible in a“true” cross-sectional view, for illustrative clarity.

Often, similar elements may be referred to by similar numbers in variousfigures (FIGs) of the drawing, in which case, typically the last twosignificant digits may be the same, the most significant digit being thenumber of the drawing figure (FIG). Furthermore, for clarity, somereference numbers may be omitted in certain drawings.

FIG. 1 shows a top-down view of a semiconductor structure in accordancewith embodiments of the present invention.

FIG. 2 shows a side view of a semiconductor structure at a startingpoint for embodiments of the present invention.

FIG. 3A shows a side view of a semiconductor structure after subsequentprocess steps of depositing a work function metal layer and a metal gatelayer in accordance with illustrative embodiments.

FIG. 3B shows a side view of a semiconductor structure after asubsequent process step of forming a gate cut mask in accordance withillustrative embodiments.

FIG. 3C shows a side view of a semiconductor structure after subsequentprocess steps of performing a gate cut and work function metal recess inaccordance with illustrative embodiments.

FIG. 3D shows a side view of a semiconductor structure after asubsequent process step of depositing a gate capping layer in accordancewith illustrative embodiments.

FIG. 4 shows a flowchart indicating process steps for embodiments of thepresent invention in accordance with illustrative embodiments.

DETAILED DESCRIPTION

It will be appreciated that this disclosure may be embodied in manydifferent forms and should not be construed as limited to the exemplaryembodiments set forth herein. Rather, these exemplary embodiments areprovided so that this disclosure will be thorough and complete and willfully convey the scope of this disclosure to those skilled in the art.The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of this disclosure.For example, as used herein, the singular forms “a”, “an”, and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. Furthermore, the use of the terms “a”, “an”, etc.,do not denote a limitation of quantity, but rather denote the presenceof at least one of the referenced items. It will be further understoodthat the terms “comprises” and/or “comprising”, or “includes” and/or“including”, when used in this specification, specify the presence ofstated features, regions, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, regions, integers, steps, operations, elements,components, and/or groups thereof.

Reference throughout this specification to “one embodiment,” “anembodiment,” “embodiments,” “exemplary embodiments,” or similar languagemeans that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” “in embodiments” and similar languagethroughout this specification may, but do not necessarily, all refer tothe same embodiment.

The terms “overlying” or “atop”, “positioned on” or “positioned atop”,“underlying”, “beneath” or “below” mean that a first element, such as afirst structure (e.g., a first layer), is present on a second element,such as a second structure (e.g., a second layer), wherein interveningelements, such as an interface structure (e.g., interface layer), may bepresent between the first element and the second element.

FIG. 1 shows a top-down view of a semiconductor structure 100 inaccordance with embodiments of the present invention. Semiconductorstructure 100 comprises semiconductor substrate 118. In embodiments,semiconductor substrate 118 comprises a silicon substrate. The substrate118 may be a bulk substrate such as a silicon wafer. Alternatively,substrate 118 may be a thinner semiconductor-on-insulator (SOI)substrate. A plurality of fins 104 are formed on the semiconductorsubstrate 118. A plurality of gates 102 (indicated generally as 102) areformed on semiconductor substrate 118. Gate cut regions, indicatedgenerally by reference 106, indicate regions where the gates 102 are tobe cut (severed) at various locations to form independent transistors.The gate cut regions 106 may be represented in an EDA (Electronic DesignAutomation) system. The independent transistors may be connectedtogether by local interconnect methods and/or back end of line (BEOL)metallization layers to form integrated circuits, such as SRAM devices.The EDA system may enforce a design rule that determines how much gatearea is needed at the end of the device. For example, in the case of cutregion 106A, a design rule may establish a distance of X between fin104A and gate cut region 106A. This extra region of gate beyond the finis referred to as the gate endcap. Gate endcap 107A has a distance X, asdetermined by the position of the gate cut region 106A. In someembodiments, distance X may range from about 20 nanometers to about 30nanometers. It is desirable to reduce the gate endcap distance as muchas possible. This can improve circuit density. In an application such asan SRAM device, where there are many similar transistors in an array,the improvement in density can be significant. However, there arechallenges to reducing the gate endcap distance. In particular, the workfunction metal layer used in a metal gate stack can vary in thickness atthe gate endcap, causing unwanted variation in the threshold voltage ofthe device. Embodiments of the present invention serve to mitigate thisproblem, while allowing a reduced gate endcap distance to improvecircuit density while achieving reduced device variability. For example,gate endcap 1078 has a distance Y between fin 1048 and gate cut region1068. In some embodiments, distance Y may range from about 5 nanometersto about 15 nanometers, which represents a considerable space savingscompared to the gate endcap distance X of gate endcap 107A.

FIG. 2 shows a side view of a semiconductor structure 200 at a startingpoint for embodiments of the present invention (similar to as viewedalong line A-A′ of FIG. 1). Semiconductor structure 200 comprisessemiconductor substrate 218. A plurality of fins (indicated generally as204) are formed on the semiconductor substrate 218. A dummy gate 220 isformed on the substrate 218 and over the plurality of fins 204. Inembodiments, the dummy gate 220 may be comprised of polysilicon. Thedummy gate may start as a blanket layer of polysilicon, which is not yetcut, and thus has a length L which may be the length of an entirecircuit block or die. In some embodiments, length L may range from about2 millimeters to about 4 millimeters. In embodiments, the length L ofthe gate line is at least two orders of magnitude larger than a gatethat has been cut. A sacrificial layer 224 is deposited on the blanketpolysilicon. In embodiments, the sacrificial layer is comprised ofsilicon nitride. The blanket polysilicon is then formed on a dummy gate220 by removing a portion of the blanket polysilicon. Spacers 226 areformed around the dummy gate 220. In embodiments, the spacers 226 arealso comprised of silicon nitride. The spacers serve to protect thedummy gate 220 from any unwanted epitaxial growth or other damage duringthe fabrication process. The sacrificial layer 224 has a thickness D. Inembodiments, the thickness D ranges from about 25 nanometers to about 35nanometers. This is thinner than is possible with prior art methods.Since the dummy gate 220 is not yet cut, the spacers 226 are disposedonly at the distal ends of the dummy gate 220. Thus, these spacers maybe far away (at a nano scale) from where the actual devices will beformed. Hence, the thinner sacrificial layer 224 is possible, because ifthe spacers 226 get damaged and expose some of the dummy gate 220, it isonly at the far ends of the dummy gate 220, and not near wherefunctional circuits are formed. The ability to use a thinner sacrificiallayer provides advantages during implantation, as it reduces undesirableshadow effects that can occur with a thicker (taller) sacrificial layerdisposed on the dummy gate 220.

FIG. 3A shows a side view of a portion of a semiconductor structure 300after subsequent process steps of depositing a metal gate stack 329 inaccordance with illustrative embodiments. The metal gate stack 329comprises a work function metal layer 330 and a metal gate layer 332.The metal gate layer 332 may be formed using a replacement metal gate(RMG) process. In such a process, the dummy gate 220 (FIG. 2) isselectively removed, and a metal gate stack is formed in the locationwhere the dummy gate used to be. To form the metal gate stack 329, thework function metal layer 330 is conformally disposed on the fins 304,followed by a metal fill layer 332. In embodiments, the metal fill layer332 may be comprised of tungsten. In embodiments, the work functionmetal layer 330 may comprise one or more metals. In some embodiments,the work function metal layer 330 may comprise titanium or atitanium-containing material. In some embodiments, the work functionmetal layer 330 may comprise aluminum or an aluminum-containingmaterial.

FIG. 3B shows a side view of semiconductor structure 300 after asubsequent process step of forming a gate cut mask 334 in accordancewith illustrative embodiments. The gate cut mask 334 may be formed usingindustry-standard patterning and lithographic techniques.

FIG. 3C shows a side view of semiconductor structure 300 aftersubsequent process steps of performing a gate cut and work functionmetal recess in accordance with illustrative embodiments. The gate cutand work function metal recess may utilize one or more anisotropic etchprocesses, such as a reactive ion etch process. As a result of the gatecut, voids 336 are formed in the gate metal fill layer 332 and workfunction metal layer 330.

FIG. 3D shows a side view of semiconductor structure 300 after asubsequent process step of depositing a gate capping layer 338 inaccordance with illustrative embodiments. The capping layer 338 isdisposed on the top and side portions of the gate stack. In embodiments,the capping layer 338 comprises silicon nitride. In other embodiments,the capping layer 338 comprises silicon oxide. In some embodiments (suchas with a silicon nitride capping layer), the capping layer is depositedusing an atomic layer deposition (ALD) process. In other embodiments(such as with a silicon oxide capping layer), the capping layer isdeposited using a spin-on deposition process. As can be seen in FIG. 3D,the work function metal layer 330 terminates at the capping layer 338 onthe semiconductor substrate 318, as indicated by termination boundary337. This arrangement provides a consistent shape for work functionmetal layer 330 with relation to the outermost fins (304A and 304B).With the consistent shape, the threshold voltage Vt of transistorsformed using fin 304A and 304B is consistent, while the gate endcapdistance is significantly reduced over prior art methods, providing animprovement in circuit density. Note that while the examples illustratedin FIGS. 3A-3D show multiple fins, embodiments of the present inventionmay also be utilized with finFETs having a single fin. Single-fin finFETdevices are used in a variety of applications, such as SRAM chips, whichmay contain hundreds of millions of single-fin finFET devices. Thus,embodiments of the present invention, which allow a reduced gate endcapdistance for each finFET, can provide a considerable improvement incircuit density for an SRAM device. In addition, in logic cellboundaries and n-p well boundaries, the ability to reduce the gateendcap distance also allows reduction in these boundary distances andoffers considerable improvement in overall logic density. In someembodiments, the capping layer 338 disposed on the side portions of thegate stack is disposed at a distance G from the fin 3048, where G rangesfrom about 5 nanometers to about 15 nanometers. This is a considerableimprovement as compared to prior art devices and techniques.

FIG. 4 shows a flowchart 400 indicating process steps for embodiments ofthe present invention in accordance with illustrative embodiments. Inprocess step 450, semiconductor fins are formed on a semiconductorsubstrate. The semiconductor fins may be formed of silicon, silicongermanium, or other suitable material. The semiconductor fins may beformed using a sidewall image transfer (SIT) technique, or othersuitable technique. In process step 451, a sacrificial protective layeris deposited on a blanket dummy gate material. In embodiments, thesacrificial protective layer is silicon nitride and the blanket dummygate material is polysilicon. In process step 452, a dummy gate isformed by removing a portion of the blanket polysilicon. In embodiments,in subsequent process steps, the dummy gate is replaced with a metalgate stack using a replacement metal gate (RMG) process. The dummy gatemay be fairly long (on the order of millimeters), and is not cut at thistime. In process step 454, spacers are formed adjacent to the dummygate. The spacers may be comprised of silicon nitride, and may bedeposited via a chemical vapor deposition (CVD) process, or othersuitable process. In process step 456, source/drain regions are formed.This may include introducing dopants, formation of epitaxial regions,silicide regions, and other process steps in accordance withindustry-standard procedures. In process step 458, the dummy gate isremoved. In embodiments, this may be performed using a selective etchprocess that is selective to polysilicon. In process step 460, a workfunction metal layer is deposited. In embodiments, the work functionmetal layer may include more than one metal sublayer. In embodiments,the work function metal layer contains titanium. In other embodiments,the work function metal layer may contain aluminum. Other work functionmetal layer materials are possible. In process step 462, a gate fillmetal layer is deposited. In embodiments, the gate fill metal layer iscomprised of tungsten. Other gate fill metal layers, such as aluminumand/or copper are possible. In process step 464, the gate metal may berecessed to an appropriate height for the gate. In process step 466, agate cut mask is formed on the gate fill metal layer. In process step468, the gate fill metal is cut. This may be performed using a selectiveanisotropic etch process for the fill metal, such as a reactive ionetch. In process step 470, if necessary, the work function metal isrecessed such that it is flush with the gate fill metal layer. Inprocess step 472, a gate capping layer is deposited on the tops andsides of the gate. The work function metal terminates at the cappinglayer on the semiconductor substrate.

While the invention has been particularly shown and described inconjunction with exemplary embodiments, it will be appreciated thatvariations and modifications will occur to those skilled in the art. Forexample, although the illustrative embodiments are described herein as aseries of acts or events, it will be appreciated that the presentinvention is not limited by the illustrated ordering of such acts orevents unless specifically stated. Some acts may occur in differentorders and/or concurrently with other acts or events apart from thoseillustrated and/or described herein, in accordance with the invention.In addition, not all illustrated steps may be required to implement amethodology in accordance with the present invention. Furthermore, themethods according to the present invention may be implemented inassociation with the formation and/or processing of structuresillustrated and described herein as well as in association with otherstructures not illustrated. Therefore, it is to be understood that theappended claims are intended to cover all such modifications and changesthat fall within the true spirit of the invention.

What is claimed is:
 1. A method of forming a semiconductor structure,comprising: forming a dummy gate on a semiconductor substrate;depositing a sacrificial layer on the dummy gate; forming spacersadjacent to the dummy gate; removing the dummy gate and sacrificiallayer; depositing a work function metal layer; depositing a fill metallayer; performing a gate cut; and depositing a gate capping layer. 2.The method of claim 1, wherein depositing a gate capping layer comprisesdepositing a silicon nitride layer.
 3. The method of claim 2, whereindepositing a gate capping layer is performed using an atomic layerdeposition process.
 4. The method of claim 1, wherein depositing a fillmetal layer comprises depositing tungsten.
 5. The method of claim 1,wherein depositing a work function metal layer comprises depositingtitanium.
 6. The method of claim 1, wherein depositing a work functionmetal layer comprises depositing aluminum.
 7. The method of claim 1,wherein forming a dummy gate on a semiconductor substrate comprisesdepositing polysilicon.
 8. The method of claim 1, wherein depositing asacrificial layer on the dummy gate comprises depositing a siliconnitride layer.
 9. The method of claim 8, wherein depositing a siliconnitride layer comprises depositing a silicon nitride layer having athickness ranging from about 25 nanometers to about 35 nanometers.
 10. Asemiconductor structure, comprising: a semiconductor substrate; a finformed on the semiconductor substrate; and a gate stack formed over thefin, wherein the gate stack comprises: a work function metal layerdisposed on the fin; a metal fill layer disposed on the work functionmetal layer; and a capping layer disposed on a top and side portions ofthe gate stack, wherein the work function metal layer terminates at thecapping layer on the semiconductor substrate.
 11. The semiconductorstructure of claim 10, wherein the metal fill layer comprises tungsten.12. The semiconductor structure of claim 10, wherein the work functionmetal layer comprises titanium.
 13. The semiconductor structure of claim10, wherein the work function metal layer comprises aluminum.
 14. Thesemiconductor structure of claim 10, wherein the capping layer comprisessilicon nitride.
 15. The semiconductor structure of claim 10, whereinthe capping layer comprises silicon oxide.
 16. A semiconductorstructure, comprising: a semiconductor substrate; a fin formed on thesemiconductor substrate; and a gate stack formed over the fin, whereinthe gate stack comprises: a work function metal layer disposed on thefin; a metal fill layer disposed on the work function metal layer; and acapping layer disposed on a top and side portions of the gate stackwherein the work function metal layer terminates at the capping layer onthe semiconductor substrate, and wherein the capping layer disposed onthe side portions of the gate stack is disposed at a distance from thefin ranging from about 5 nanometers to about 15 nanometers.
 17. Thesemiconductor structure of claim 16, wherein the metal fill layercomprises tungsten.
 18. The semiconductor structure of claim 16, whereinthe work function metal layer comprises titanium.
 19. The semiconductorstructure of claim 16, wherein the work function metal layer comprisesaluminum.
 20. The semiconductor structure of claim 16, wherein thecapping layer comprises silicon nitride.